Multilayer tubular structure having better resistance to extraction in biofuel and use thereof

ABSTRACT

A multilayer tubular structure (MLT) for transport of alcohol-containing petrol. The MLT from the outside inwards has at least one barrier layer and at least one inner layer located below the barrier layer. The inner layer, or all the layers contain on average at most 1.5% of plasticizer relative to the total weight of the composition of each layer or to the total weight of all the compositions of the layers. The inner layer is predominantly made of at least one polyamide having more than 75% aliphatic units. The aliphatic polyamide has a mean number of carbon atoms per nitrogen atom selected from the following:A: from 4 to 8.5;B: from 7 to 10;C: from 9 to 18;with the proviso that when the inner layer comprises at least three polyamides, at least one of the polyamides A, B and C is excluded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the national phase of International Application No.PCT/FR2017/050066, filed 12 Jan. 2017, which claims priority to FrenchApplication No. 1650337, filed 15 Jan. 2016. The disclosure of each ofthese applications is incorporated herein by reference in its entiretyfor all purposes.

FIELD OF INVENTION

The invention relates to a multilayer structure, in particular in theform of a tube, and the use thereof for transporting fluids, inparticular fuel of petrol type, especially alcohol-containing petrol, inparticular for motor vehicles.

The invention more particularly relates to the tubes present within anengine. These tubes may for example be intended for the transport offuels, especially between the tank and the engine, for the coolingcircuit, for the hydraulics system, or else intended for the airconditioning circuit or the transport of urea/water mixture.

BACKGROUND OF INVENTION

For the transport of petrol, and in particular of bio-petrol, numerouscriteria must be met, in particular good barrier properties (forenvironmental protection reasons), cold impact, pressure resistance,etc.

For reasons of safety and environmental conservation, especially withthe advent of new biofuels, car manufacturers are demanding particularmechanical characteristics for the abovementioned tubes, and alsocharacteristics of very low permeability and good resistance to thedifferent constituents of the fuels, the latter varying depending on thecountry (hydrocarbons, additives, alcohols such as methanol and ethanol,alcohols possibly being predominant components in some cases), to enginelubrication oils, and to other chemical products liable to beencountered in this environment (battery acids, brake fluids, coolantliquids, metal salts such as zinc or calcium chloride).

The characteristics of the specifications commonly required by carmanufacturers for a tube to be deemed satisfactory are, cumulatively,the following:

-   -   good, lasting adhesion between the layers, if the tube is a        multilayer tube, most particularly after having been exposed to        the fuel;    -   good integrity of the connections (tubes with connectors) after        circulation of the fuel, that is to say not leading to any        leakage;    -   good dimensional stability of the tube when it is used with        petrol;    -   good impact resistance under cold conditions (from −30° C. to        −40° C. approximately), such that the tube does not shatter;    -   good heat resistance (approximately 150° C.), such that the tube        does not deform;    -   good resistance to ageing in a hot oxidative medium (for        example: hot air from the engine compartment, from 100 to        150° C. approximately);    -   good resistance to fuels and to their degradation products, and        especially with high peroxide contents;    -   very low permeability to fuels, and more particularly good        barrier properties to biofuels, both for the polar components        thereof (such as ethanol) and for the apolar components thereof        (hydrocarbons);    -   good flexibility of the tube to facilitate assembly, especially        of the fuel supply pipe;    -   good resistance to ZnCl₂ (for example in winter when the roads        have been gritted, the outside of the tube being exposed to this        environment).

Moreover, the desired tubes must avoid the following drawbacks:

-   -   if the tube is a multilayer tube, the delamination of the        layers, especially the inner layers, especially during connector        insertion (which may lead to leakages);    -   excessive swelling of the tube after ageing in petrol/diesel        systems (including for biodiesels or bio-petrols), which may        lead to leakages or problems of positioning under the vehicle.

Recently, a new problem has appeared, namely the excess of extractablematter from the multilayer tube following prolonged contact with thealcohol-containing petrol. This extractable matter is liable to block orclog the injectors of the vehicle engines. Car manufacturers, inparticular Volkswagen, have therefore established new criteria forselecting tubes able to transport petrol, especially alcohol-containingpetrol, in motor vehicles, which are stricter than they used to be.Thus, the new test developed by different manufacturers, especiallyVolkswagen, consists in determining the proportion of extractables of apetrol transport tube after bringing the inside thereof into contactwith hot alcohol-containing petrol for several hours and weighing theevaporation residue of the petrol contained within the tube,corresponding to the extractables. The tube tested may thus only be usedfor transporting petrol if the proportion of extractables is as low aspossible, in particular less than or equal to 6 g/m² (of tube innersurface area).

Currently, there are two types of tubes: single-layer and multilayertubes, that is to say consisting of one or more layers of polymer. Forthe transport of petrol, in particular, the use of a multilayer tubecomprising a barrier layer is tending to become widespread forecological reasons.

Conventionally, the tubes used are produced by mono-extrusion, if it isa single-layer tube, or by coextrusion of the different layers, if it isa multilayer tube, according to customary techniques for transformingthermoplastics.

The structures (MLT) for transporting petrol typically consist of abarrier layer such as EVOH, surrounded on both sides by a PA layer (atleast one layer) and optionally comprising layers of binders in theevent that the adhesion between the other layers proves insufficient.

Thus, patent EP 2098580 especially describes tubes having an EVOHbarrier and at least two layers of polyamides, plasticized orunplasticized, one being located above the barrier layer and the otherbeing located below the barrier layer.

Nonetheless, this type of structure, and also other MLTs known to thoseskilled in the art, are no longer suited to the abovementioned new testfor extractables.

SUMMARY OF THE INVENTION

The present invention aims to solve this new problem by a specificarrangement and composition of the layers of the multilayer structure.

The present invention relates to a multilayer tubular structure (MLT)intended for the transport of fluids, in particular of petrol,especially alcohol-containing petrol, comprising, from the outsideinwards, at least one barrier layer (1) and at least one inner layer (2)located below the barrier layer,

-   -   said inner layer (2), or all the layers (2) and the other        optional layers located below the barrier layer, containing on        average from 0 to 1.5% by weight of plasticizer relative to the        total weight of the composition of the layer (2) or to the total        weight of all the compositions of the layers (2) and the other        optional layers located below the barrier layer, respectively,    -   said inner layer (2) predominantly comprising at least one        polyamide of aliphatic type or consisting of more than 75% of        aliphatic units, said aliphatic polyamide being chosen from:        -   a polyamide denoted A, having a mean number of carbon atoms            per nitrogen atom, denoted C_(A), of from 4 to 8.5,            advantageously from 4 to 7;        -   a polyamide denoted B, having a mean number of carbon atoms            per nitrogen atom, denoted C_(B), of from 7 to 10,            advantageously from 7.5 to 9.5;        -   a polyamide denoted C, having a mean number of carbon atoms            per nitrogen atom, denoted C_(C), of from 9 to 18,            advantageously from 10 to 18;    -   with the proviso that when said inner layer (2) comprises at        least three polyamides, at least one of said polyamides A, B and        C is excluded.

It would not be a departure from the context of the invention if thesubject, intended for transporting fluid, also served for storingfluids.

DETAILED DESCRIPTION OF THE INVENTION

The term “fluid” denotes a gas used in a motor vehicle, or a liquid, inparticular a liquid, and especially an oil, a brake fluid, a ureasolution, a glycol-based coolant liquid, fuels, especially light fuelsliable to pollute, advantageously fuels other than diesel, especiallypetrol or LPG, in particular petrol and more particularlyalcohol-containing petrol.

Air, nitrogen and oxygen are excluded from the definition of said gas.

Advantageously, said fluid denotes fuels, in particular petrol,especially alcohol-containing petrol.

The term “petrol” denotes a mixture of hydrocarbons originating from thedistillation of petroleum, to which may be added additives or alcoholssuch as methanol or ethanol, alcohols possibly being predominantcomponents in some cases.

The expression “alcohol-containing petrol” denotes a petrol to whichmethanol or ethanol have been added. It also denotes an E95 type petrol,which does not contain any petroleum distillation products.

The expression “all the layers (2) and the other optional layers locatedbelow the barrier layer” means all the layers present, located below thebarrier layer.

The expression “barrier layer” denotes a layer having characteristics oflow permeability and of good resistance to the various constituents ofthe fluids, especially the fuels, that is to say that the barrier layerslows the passage of the fluid, especially of the fuel, both in terms ofthe polar components thereof (such as ethanol) and the apolar componentsthereof (hydrocarbons) into the other layers of the structure or even tothe outside of the structure. The barrier layer is therefore a layerwhich first and foremost makes it possible not to lose too much petrolinto the atmosphere by diffusion, thereby making it possible to avoidatmospheric pollution.

These barrier materials may be polyamides with a low carbon content,that is to say in which the mean number of carbon atoms (C) relative tothe nitrogen atom (N) is less than 9, which are preferablysemicrystalline and have a high melting point, polyphthalamides, and/oralso non-polyamide barrier materials such as highly crystalline polymerssuch as the copolymer of ethylene and vinyl alcohol (denoted EVOHhereinafter), or even functionalized fluoro materials such asfunctionalized polyvinylidene fluoride (PVDF), the functionalizedcopolymer of ethylene and tetrafluoroethylene (ETFE), the functionalizedcopolymer of ethylene, tetrafluoroethylene and hexafluoropropylene(EFEP), functionalized polyphenylene sulphide (PPS) or functionalizedpolybutylene naphthalate (PBN). If these polymers are notfunctionalized, then it is possible to add an intermediate layer ofbinder to ensure good adhesion within the MLT structure.

Among these barrier materials, EVOHs are particularly beneficial,especially those containing the most vinyl alcohol comonomer, and alsothose which have been impact-modified, since they make it possible toproduce less fragile structures.

The inventors have thus found that the absence of, or at least a verysmall proportion of, plasticizer in the inner layer(s), that is to saythe layer(s) located under the barrier layer, made it possible togreatly reduce the proportion of extractables as determined by a test asdefined above, and in particular by a test which consists in filling atubular structure with alcohol-containing petrol of FAM B type and inheating everything at 60° C. for 96 hours, then in emptying it byfiltering it into a beaker, then in leaving the filtrate of the beakerto evaporate at room temperature, to finally weigh this residue, theproportion of which must be less than or equal to approximately 6 g/m2of tube inner surface area.

The alcohol-containing petrol FAM B is described in standards DIN51604-1: 1982, DIN 51604-2: 1984 and DIN 51604-3: 1984.

Briefly, the alcohol-containing petrol FAM A is firstly prepared with amixture of 50% of toluene, 30% of isooctane, 15% of diisobutylene and 5%of ethanol then FAM B is prepared by mixing 84.5% of FAM A with 15% ofmethanol and 0.5% of water.

FAM B consists in total of 42.3% of toluene, 25.4% of isooctane, 12.7%of diisobutylene, 4.2% of ethanol, 15% of methanol and 0.5% of water.

When a single layer (2) is present, the latter is in contact with thefluid.

In the event that several layers (2) are present, it is possible thatone of the inner layers has a proportion of plasticizer greater than1.5% by weight, but in this case the proportion of plasticizer beyond1.5% is compensated by the thickness of the layer which is then muchthinner, such that the mean value of plasticizer present in all theinner layers does not exceed 1.5%. The proportion of plasticizer in thislayer may then be up to 15%, but its thickness does not then exceed 10%of the total thickness of the tube; preferably, it does not exceed 100μm.

This much thinner layer may be either directly in contact with thebarrier layer, or the innermost layer which is then in contact with thefluid.

The expression “said inner layer (2) predominantly comprising at leastone polyamide of aliphatic type” means that said polyamide of aliphatictype is present in a proportion of more than 50% by weight in the layer(2). The polyamide of aliphatic type is linear and is not ofcycloaliphatic type.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (2) also predominantly comprises aliphatic units, namely morethan 50% of aliphatic units.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (2) consists of more than 75% of aliphatic units; preferablysaid predominant polyamide of aliphatic type of the layer(s) (2) istotally aliphatic.

Among these barrier materials, PPAs are also beneficial, especiallycoPA6T, PA9T and the copolymers thereof, and PA10T and the copolymersthereof.

According to the present application, the term “polyamide”, also denoted“PA”, covers:

-   -   homopolymers,    -   copolymers, or copolyamides, based on various amide units, such        as, for example, copolyamide 6/12 with amide units derived from        lactam 6 and lactam 12,    -   alloys of polyamides, as long as the polyamide is the        predominant constituent.

There is also a category of copolyamides in the broad sense, which,while not preferred, form part of the context of the invention. Theseare copolyamides comprising not only amide units (which will bepredominant, hence the fact that they are to be considered ascopolyamides in the broad sense), but also units which are not amide innature, for example ether units. The most well known examples are PEBAsor polyether-block-amides, and the copolyamide-ester-ether,copolyamide-ether and copolyamide-ester variants thereof. Among these,mention will be made of PEBA-12, in which the polyamide units are thesame as those of PA12, and PEBA-6.12, in which the polyamide units arethe same as those of PA6.12.

Homopolyamides, copolyamides and alloys are also distinguished from oneanother by their number of carbon atoms per nitrogen atom, with theknowledge that there are as many nitrogen atoms as amide groups(—CO—NH—).

A polyamide with a high carbon content is a polyamide with a highcontent of carbon atoms (C) relative to the nitrogen atom (N). These arepolyamides with approximately at least 9 carbon atoms per nitrogen atom,such as, for example, polyamide-9, polyamide-12, polyamide-11,polyamide-10.10 (PA10.10), copolyamide 12/10.T, copolyamide 11/10.T,polyamide-12.T, and polyamide-6.12 (PA6.12). T represents terephthalicacid.

The nomenclature used to define polyamides is described in the standardISO 1874-1:1992 “Plastics—Polyamide (PA) molding and extrusionmaterials—Part 1: Designation”, especially on page 3 (tables 1 and 2),and is well known to those skilled in the art.

A polyamide with a low carbon content is a polyamide with a low contentof carbon atoms (C) relative to the nitrogen atom (N). These arepolyamides with approximately less than 9 carbon atoms per nitrogenatom, such as, for example, polyamide-6, polyamide-6.6, polyamide-4.6,copolyamide-6.T/6.6, copolyamide 6.I/6.6, copolyamide 6.T/6.I/6.6, andpolyamide 9.T. I represents isophthalic diacid.

In the case of a homopolyamide of PA-X.Y type, with X denoting a unitobtained from a diamine and Y denoting a unit obtained from a diacid,the number of carbon atoms per nitrogen atom is the mean of the numbersof carbon atoms present in the unit derived from the diamine X and inthe unit derived from the diacid Y. Thus, PA6.12 is a PA with 9 carbonatoms per nitrogen atom, in other words a C9 PA. PA6.13 is C9.5.

In the case of the copolyamides, the number of carbon atoms per nitrogenatom is calculated according to the same principle. The calculation ismade on a molar pro rata basis of the various amide units. In the caseof a copolyamide having units not of amide type, the calculation is onlymade on the amide unit portion. Thus, for example, for PEBA-12, which isa block copolymer of 12 amide units and of ether units, the mean numberof carbon atoms per nitrogen atom will be 12, as for PA12; forPEBA-6.12, it will be 9, as for PA6.12.

Thus, polyamides with a high carbon content such as the polyamide PA12or 11, adhere with difficulty to an EVOH polymer, to a polyamide with alow carbon content such as the polyamide PA6, or else to an alloy ofpolyamide PA6 and polyolefin (such as, for example, an Orgalloy® sold byArkema).

Nonetheless, it is observed that the currently proposed tube structuresare unsatisfactory for a use intended for biofuels, since the demands ofthe car manufacturers' specifications, recalled above, cannot all besimultaneously met.

Biofuels are not solely derived from petroleum but comprise a proportionof polar products such as alcohols of plant origin, such as ethanol ormethanol, of at least 3%. This content may be as high as 85% or even95%.

In addition, the circulation temperature of the fuel tends to rise, dueto the new engines (more confined, operating at a higher temperature).

In one advantageous embodiment, said inner layer (2), or each of thelayers (2) and the other optional layers located below the barrierlayer, contains from 0 to 1.5% by weight of plasticizer relative to thetotal weight of the composition of the layer (2) or to the total weightof each of the compositions of the layers (2) and the other optionallayers located below the barrier layer, respectively.

In one advantageous embodiment, in the multilayer tubular structure(MLT) as defined above, said inner layer (2), or each of the layers (2)and the other optional layers located below the barrier layer, does not(do not) contain plasticizer.

In this embodiment, all the layers located below the barrier layer donot contain any plasticizer at all and consist of one of the preferredstructures of the invention.

In one advantageous embodiment, the present invention relates to amultilayer tubular structure (MLT) as defined above, in which at leastone layer (3) further outwards, located above the barrier layer, ispresent, said outer layer (3) predominantly comprising at least onepolyamide of aliphatic type or consisting of more than 75% of aliphaticunits, in particular said aliphatic polyamide having a mean number ofcarbon atoms per nitrogen atom of from 9.5 to 18, advantageously from 11to 18.

The expression “said outer layer (3) predominantly comprising at leastone polyamide of aliphatic type” means that said polyamide of aliphatictype is present in a proportion of more than 50% by weight in the layer(3). The polyamide of aliphatic type is linear and is not ofcycloaliphatic type.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (3) also predominantly comprises aliphatic units, namely morethan 50% of aliphatic units.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (3) consists of more than 75% of aliphatic units; preferablysaid predominant polyamide of aliphatic type of the layer(s) (3) istotally aliphatic.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (2) and of the layer(s) (3) also predominantly comprisesaliphatic units, namely more than 50% of aliphatic units.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (2) and of the layer(s) (3) consists of more than 75% ofaliphatic units; preferably said predominant polyamide of aliphatic typeof the layer(s) (2) and of the layer(s) (3) is totally aliphatic.

Advantageously, the present invention relates to a multilayer tubularstructure (MLT) as defined above, in which said outer layer (3)comprises from 0 to 15% of plasticizer relative to the total weight ofthe composition of the layer (3), or in which all the outer layerscomprise on average from 0 to 5% of plasticizer.

It is possible to have a greater proportion of plasticizer in the outerlayer(s), that is to say the layer(s) located above the barrier layer,without significantly increasing the proportion of extractables.

As already indicated above for the layers (2), in the event that severallayers (3) are present, it is possible that one of the outer layers hasa greater proportion of plasticizer, such as 15% by weight, but in thiscase the proportion of plasticizer is compensated by the thickness ofthe layer which is then much thinner, such that the mean value ofplasticizer present in all the outer layers does not exceed 5%. Theproportion of plasticizer in this layer may then be up to 15%, but itsthickness does not exceed 20% of the total thickness of the tube;preferably, it does not exceed 200 μm.

Advantageously, the present invention relates to a multilayer tubularstructure (MLT) comprising a layer (3) as defined above, in which atleast one second outer layer (3′) located above the barrier layer ispresent, and is preferably located above the layer (3), said layer (3′)being plasticized, said plasticizer being in particular present in aproportion from 1.5% to 15% by weight relative to the total weight ofthe composition of said layer, the thickness of said layer (3′)preferably represents up to 20% of the total thickness of the tubularstructure, in particular up to 200 μm.

The layer (3′), just like the layer (3), predominantly comprises apolyamide of aliphatic type, that is to say that said polyamide ofaliphatic type is present in a proportion of more than 50% by weight inthe layer (3′). The polyamide of aliphatic type is linear and is not ofcycloaliphatic type.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (3′) also predominantly comprises aliphatic units, namely morethan 50% of aliphatic units.

Advantageously, said predominant polyamide of aliphatic type of thelayer(s) (3′) consists of more than 75% of aliphatic units; preferablysaid predominant polyamide of aliphatic type of the layer(s) (3′) istotally aliphatic.

In another embodiment, the present invention relates to a multilayertubular structure (MLT), in which the layer(s) (3) comprise(s) up to1.5% by weight of plasticizer relative to the total weight of thecomposition of said layer or of all the compositions of the layers (3).

Advantageously, the multilayer tubular structure (MLT) comprises asingle layer (3) and does not contain plasticizer.

Advantageously, the multilayer tubular structure (MLT) comprises asingle layer (3) and a single layer (2), the layers (2) and (3)containing no plasticizer.

In another embodiment, the present invention relates to a multilayertubular structure (MLT), in which the plasticizer content of all thelayers located above the barrier layer is at most 5% by weight relativeto the total weight of the compositions of all the layers located abovethe barrier layer.

In another embodiment, the present invention relates to a multilayertubular structure (MLT), in which the layer (3′) is the outermost layerand the only layer which is plasticized, the layer(s) (3) not containingplasticizer.

The proportion of plasticizer may represent up to 15% by weight of thetotal weight of the composition of the layer (3′). The greater theproportion of plasticizer, the thinner the layer (3′) will be, with athickness of said layer (3′) which preferably represents up to 20% ofthe total thickness of the tubular structure, in particular up to 200μm.

Advantageously, the multilayer tubular structure (MLT) consists of fourlayers, from the outside inwards, (3′)//(3)//(1)//(2), the layer (3′)being the only plasticized layer, in proportions as defined above, thelayer (3) and the layer (2) containing no plasticizer.

A multilayer tubular structure (MLT) consisting of four layers, from theoutside inwards, (3′)//(3)//(1)//(2), has the advantage of having anelongation at break, at t=0 when the structure is very dry and with avery low degree of humidity of between 0 and 30% relative humidity,which is very good, and especially better than a structure in which thelayers (3′), (3) and (2) do not contain plasticizer.

Advantageously, in the latter embodiment, the layer (3′) is theoutermost layer and the polyamide of the latter is a long-chainpolyamide, i.e. a mean number of carbon atoms per nitrogen atom, denotedCc, of from 9.5 to 18, the layer (3) is located between the barrierlayer and the layer (3′) and the polyamide of this layer (3) is ashort-chain polyamide, i.e. a mean number of carbon atoms per nitrogenatom, denoted Ca, of from 4 to 9.

Advantageously, in the latter embodiment, with the layer (3′) at athickness of from 100 to 200 μm, the layer (3) has a thickness of atleast 200 μm and the layer (1) has a thickness of from 100 to 200 μm.

Advantageously, in the latter embodiment, the layer (3′) is theoutermost layer and the polyamide of the latter is a long-chainpolyamide, i.e. a mean number of carbon atoms per nitrogen atom, denotedCc, of from 9.5 to 18, the layer (3) is located between the barrierlayer and the layer (3′) and the polyamide of this layer (3) is ashort-chain polyamide, i.e. a mean number of carbon atoms per nitrogenatom, denoted Ca, of from 4 to 9, with the layer (3′) at a thickness offrom 100 to 200 μm, the layer (3) has a thickness of at least 200 μm andthe layer (1) has a thickness of from 100 to 200 μm.

Advantageously, the multilayer tubular structure (MLT) consists of fivelayers, from the outside inwards, (3′)//(3)//(1)//(2)//(2′), the layer(3′) being the only plasticized layer, in proportions as defined above,the layer (3) and the layers (2) and (2′) containing no plasticizer, thelayer (2′) being a polyamide as defined for the layer (2) but differentfrom that of the layer (2). This type of structure makes it possible toincrease the elongation at break under very low humidity conditions,without excessively stiffening the structure.

Regardless of the number of layers: three, four, five or more, thepreferred tubular structures are those containing as little plasticizeras possible, and preferably the least amount of plasticizer in theinnermost layers, that is to say the layers closest to the fluid. Thesestructures may be the following:

-   -   Multilayer tubular structure (MLT) containing no more than 1.5%        of plasticizer in the first 50% of the thickness thereof,        starting from the inner face in contact with the fluid.    -   Multilayer tubular structure (MLT) containing no more than 1.5%        of plasticizer in the first 75% of the thickness thereof,        starting from the inner face in contact with the fluid.    -   Multilayer tubular structure (MLT) containing no more than 1.5%        of plasticizer in the first 85% of the thickness thereof,        starting from the inner face in contact with the fluid.    -   Multilayer tubular structure (MLT) containing no plasticizer in        the first 50% of the thickness thereof, starting from the inner        face in contact with the fluid.    -   Multilayer tubular structure (MLT) containing no plasticizer in        the first 75% of the thickness thereof, starting from the inner        face in contact with the fluid.    -   Multilayer tubular structure (MLT) containing no plasticizer in        the first 85% of the thickness thereof, starting from the inner        face in contact with the fluid.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which at least one layer(4) is present, said layer (4) containing no more than 15% by weight ofplasticizer, preferably no more than 1.5% by weight of plasticizerrelative to the total weight of the constituents of the layer (4);advantageously, the layer (4) does not contain plasticizer, said layer(4) predominantly comprising at least one polyamide of aliphatic type orconsisting of more than 75% of aliphatic units, said aliphatic polyamidebeing chosen from:

-   -   a polyamide denoted A, having a mean number of carbon atoms per        nitrogen atom, denoted CA, of from 4 to 8.5, advantageously from        4 to 7;    -   a polyamide denoted B, having a mean number of carbon atoms per        nitrogen atom, denoted CB, of from 7 to 10, advantageously from        7.5 to 9.5;    -   a polyamide denoted C, having a mean number of carbon atoms per        nitrogen atom, denoted CC, of from 9 to 18, advantageously from        10 to 18;

with the proviso that when said layer (4) comprises at least threepolyamides, at least one of said polyamides A, B and C is excluded,

said layer (4) being located between the barrier layer (1) and the innerlayer (2) and/or between the outer layer (3) and the barrier layer (1);

or said layer (4) is a layer of binder, the thickness of whichrepresents up to 15% of the structure (MLT).

The layer (4), when it is not a layer of binder, is a polyamide ofaliphatic type as defined for the layers (2), (3) and (3′).

Advantageously, the tubular structure of the invention is a four-layerstructure consisting, from the outside inwards, of the following layers:(3)//(4)//(1)//(2), the layer (3) being plasticized up to 15% as above,and thin, and the layer (4), when it is different from the layer ofbinder as defined above, does not contain plasticizer; nor does thelayer (2).

Advantageously, the tubular structure of the invention is a four-layerstructure consisting, from the outside inwards, of the following layers:(3)//(1)//(4)//(2), the layer (3) being plasticized up to 15% by weightas above, and preferably thin, and the layer (4), when it is differentfrom the layer of binder as defined above, does not contain plasticizer;nor does the layer (2).

Nonetheless, this layer (3) plasticized up to 15% by weight must not betoo thin, otherwise the barrier layer is not central enough and the MLTstructure risks not being good enough in terms of impact. On the otherhand, it may be very thin if there is an additional thick(non-plasticized) layer between the layer (3) and the layer (1), suchthat the layer (1) is not too off-centre.

Another layer (2′) and/or a layer (3′) may also be present in these twotypes of four-layer structures.

Said layer (4) may also be a binder as described, in particular, inpatents EP 1452307 and EP1162061, EP 1216826 and EP0428833.

It is implicit that the layers (3) and (1) or (1) and (2) adhere to oneanother. The layer of binder is intended to be interposed between twolayers which do not adhere, or which adhere with difficulty, to oneanother.

The binder may be, for example, but without being limited thereto, acomposition based on 50% of copolyamide 6/12 (70/30 weight ratio), of Mn16 000, and on 50% copolyamide 6/12 (30/70 weight ratio), of Mn 16 000,a composition based on PP (polypropylene) grafted with maleic anhydride,known under the name Admer QF551A by Mitsui, a composition based onPA610 (of Mn 30 000, and as defined elsewhere) and on 36% of PA6 (of Mn28 000) and on 1.2% of organic stabilizers (consisting of 0.8% of phenolLowinox 44625 from Great Lakes, of 0.2% of phosphite Irgafos 168 fromCiba, of 0.2% of UV stabilizer Tinuvin 312 from Ciba), a compositionbased on PA612 (of Mn 29 000, and as defined elsewhere) and on 36% ofPA6 (of Mn 28 000, and as defined elsewhere) and on 1.2% of organicstabilizers (consisting of 0.8% of phenol Lowinox 44B25 from GreatLakes, of 0.2% of phosphite Irgafos 168 from Ciba, of 0.2% of UVstabilizer Tinuvin 312 from Ciba), a composition based on PA610 (of Mn30 000, and as defined elsewhere) and on 36% of PA12 (of Mn 35 000, andas defined elsewhere) and on 1.2% of organic stabilizers (consisting of0.8% of phenol Lowinox 44B25 from Great Lakes, of 0.2% of phosphiteIrgafos 168 from Ciba, of 0.2% of UV stabilizer Tinuvin 312 from Ciba),a composition based on 40% PA6 (of Mn 28 000, and as defined elsewhere),on 40% of PA12 (of Mn 35 000, and as defined elsewhere) and on 20% offunctionalized EPR Exxelor VA1801 (Exxon) and on 1.2% of organicstabilizers (consisting of 0.8% of phenol Lowinox 44B25 from GreatLakes, of 0.2% of phosphite Irgafos 168 from Ciba, of 0.2% of UVstabilizer Tinuvin 312 from Ciba) or else a composition based on 40%PA6.10 (of Mn 30 000, and as defined elsewhere), on 40% of PA6 (of Mn 28000, and as defined elsewhere) and on 20% of impact modifier ofethylene/ethyl acrylate/anhydride type in the weight ratio 68.5/30/1.5(MFI 6 at 190° C. under 2.16 kg), and on 1.2% of organic stabilizers(consisting of 0.8% of phenol Lowinox 44B25 from Great Lakes, of 0.2% ofphosphite Irgafos 168 from Ciba, of 0.2% of UV stabilizer Tinuvin 312from Ciba).

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which a layer (4′) ispresent, said layer (4′) predominantly comprising at least one polyamideof aliphatic type or consisting of more than 75% of aliphatic units,said aliphatic polyamide being chosen from:

-   -   a polyamide denoted A, having a mean number of carbon atoms per        nitrogen atom, denoted CA, of from 4 to 8.5, advantageously from        4 to 7;    -   a polyamide denoted B, having a mean number of carbon atoms per        nitrogen atom, denoted CB, of from 7 to 10, advantageously from        7.5 to 9.5;    -   a polyamide denoted C, having a mean number of carbon atoms per        nitrogen atom, denoted CC, of from 9 to 18, advantageously from        10 to 18;

with the proviso that when said layer (4′) comprises at least threepolyamides, at least one of said polyamides A, B and C is excluded,

or said layer (4′) is a layer of binder, the thickness of whichrepresents up to 15% of the structure (MLT),

said at least one polyamide of said layer (4′) being able to beidentical or different to said polyamide of the layer (4);

said layer (4′) being located between the outer layer (3) and thebarrier layer (1) and said layer of binder (4) being located between thebarrier layer (1) and the inner layer (2).

The layer (4′) may or may not contain a plasticizer. Advantageously, itdoes not contain plasticizer, just like the layer (2) and the layer (4),the layer (3) being plasticized but thin, as defined above.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the polyamide of theinner layer (2) or the polyamide of the outer layer (3) is a totallyaliphatic polyamide, preferably the polyamide of the inner layer (2) andthe polyamide of the outer layer (3) are totally aliphatic polyamides.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which a second barrierlayer (5) is present, said second barrier layer (5) being adjacent, ornot adjacent, to the first barrier layer (1), and located below saidbarrier layer (1).

It may be beneficial, in particular for alcohol-containing petrols, andmost particularly for those containing methanol, to place a secondbarrier layer in order to further limit the diffusion of the petrol intothe atmosphere and/or to reduce the content of extractables.

This second barrier layer is different from the first barrier layer (1).

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the barrier layer (1)is a layer made of EVOH.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the EVOH is an EVOHcomprising up to 27% of ethylene.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the EVOH is an EVOHcomprising an impact modifier.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the barrier layer (1)is a layer made of polyphthalamide (PPA).

The term PPA means a composition predominantly based on a polyamidecomprising a majority of units which comprise at least one aromaticmonomer, in particular polyphthalamide of copolyamide 6.T/x type (inwhich x denotes one or more comonomers) such as Zytel HTNs from Dupont,such as Grivory HTs from Ems, such as Amodels from Solvay, such asGenestars from Kuraray, such as PPA compositions based on coPA6T/6I,coPA6T/66, coPA6T/6, on coPA6T/6I/66, on PPA9T, on coPPA9T/x, on PPA10T,or on coPPA10T/x.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the barrier layer (1)is a layer made of EVOH and the second barrier layer (5) is a layer madeof PPA or fluoropolymer, in particular of ETFE, EFEP or CPT type.

Advantageously, the barrier layer (1) is a layer made of EVOH and thesecond barrier layer (5) is a layer made of PPA.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the barrier layer (1)is a layer made of EVOH and the second barrier layer (5) is a layer madeof PPA.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the barrier layer (1)is a layer made of EVOH and the second barrier layer (5) is a layer madeof fluoropolymer, in particular of ETFE, EFEP or CPT type.

Advantageously, in the multilayer tubular structure (MLT) as definedabove, the polyamide of the inner layer (2) is a composition based on apolyamide chosen from A, B or C as defined above, in particular PA6,PA66, PA6/66, PA11, PA610, PA612 or PA1012, the correspondingcopolyamides and the mixtures of said polyamides or copolyamides, thepolyamides obtained from a lactam being advantageously washed.

Advantageously, in the multilayer tubular structure (MLT) as definedabove, the polyamide of the outer layer (3) is a polyamide chosen from Bor C as defined above, in particular PA11, PA12, PA610, PA612 or PA1012,the corresponding copolyamides and the mixtures of said polyamides orcopolyamides, the polyamides obtained from a lactam being advantageouslywashed.

Advantageously, in the multilayer tubular structure (MLT) as definedabove, the polyamide of the inner layer (2) is a composition based on apolyamide chosen from A, B or C as defined above, in particular PA6,PA66, PA6/66, PA11, PA610, PA612 or PA1012, the correspondingcopolyamides and the mixtures of said polyamides or copolyamides, thepolyamides obtained from a lactam being advantageously washed, and thepolyamide of the outer layer (3) is a polyamide chosen from B or C asdefined above, in particular PA11, PA12, PA610, PA612 or PA1012, thecorresponding copolyamides and the mixtures of said polyamides orcopolyamides, the polyamides obtained from a lactam being advantageouslywashed.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined below, in which the polyamide of theinner layer (2) or of at least one of the other layers (2) is aconductive polyamide.

When the tubular structure of the invention comprises several layers(2), the conductive layer is the innermost thereof, that is to say thatwhich is in contact with the fluid.

Advantageously, in the multilayer tubular structure (MLT) as definedabove, the polyamide of the layer (4) and/or (4′) is a mixture of apolyamide having a mean number of carbon atoms per nitrogen atom of 10or more, and a polyamide having a mean number of carbon atoms pernitrogen atom of 6 or less, for example PA12 and PA6 and ananhydride-functionalized (co)polyolefin.

Advantageously, in the multilayer tubular structure (MLT) as definedabove, the polyamide of the layer (4) and/or (4′) is chosen from thebinary mixtures: PA6 and PA12, PA6 and PA612, PA6 and PA610, PA12 andPA612, PA12 and PA610, PA1010 and PA612, PA1010 and PA610, PA1012 andPA612, PA1012 and PA610, and the ternary mixtures: PA6, PA610 and PA12;PA6, PA612 and PA12; PA6, PA614 and PA12.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which at least one of thelayers (2), (3), (3′), (4) and (4′) comprises at least one impactmodifier and/or at least one additive.

Of course, the impact modifier or the additive is not a plasticizer.

Advantageously, the layers (2) and (3) comprise at least one impactmodifier and/or at least one additive.

Advantageously, the layers (2), (3) and (3′) comprise at least oneimpact modifier and/or at least one additive.

Advantageously, the layers (2), (3), (3′) and (4′) comprise at least oneimpact modifier and/or at least one additive.

Advantageously, the layers (2), (3), (3′) (4) and (4′) comprise at leastone impact modifier and/or at least one additive.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the structurecomprises three layers in the following order: (3)//(1)//(2), the layers(3) and/or (2) containing no more than 1.5% by weight of plasticizerrelative to the total weight of the composition of each layer, inparticular the layer (3) and/or (2) does not (do not) containplasticizer.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the structurecomprises four layers in the following order: (3′)//(3)//(1)//(2), thelayer (3′) being as defined above, the layer (2) and/or (3) containingno more than 1.5% by weight of plasticizer relative to the total weightof the composition of each layer, in particular the layer (2) and/or (3)does not (do not) contain plasticizer.

In another embodiment, the present invention relates to a multilayertubular structure (MLT) as defined above, in which the structurecomprises five layers in the following order:

(3′)//(3)//(1)//(5)//(2) in which the layer (1) is a layer of EVOH, thelayer (5) is a layer of PPA, the layer (2) containing no more than 1.5%by weight of plasticizer relative to the total weight of the compositionof each layer, in particular the layer (2) does not contain plasticizer,the layer (3) and (3′) comprising plasticizer; or

(3′)//(3)//(1)//(2)//(5) in which the layer (1) is a layer of EVOH, thelayer (5) is a layer of PPA, the layer (2) containing no more than 1.5%by weight of plasticizer relative to the total weight of the compositionof each layer, in particular the layer (2) does not contain plasticizer,the layer (3) and (3′) comprising plasticizer; or

(3′)//(4)//(1)//(4)//(2) in which the layer (3) is as defined in Claim3, the layer (2) and (4) containing no more than 1.5% by weight ofplasticizer relative to the total weight of the composition of eachlayer, in particular the layer (2) and/or (4) does not (do not) containplasticizer, the layer (4′) comprising plasticizer, in particular thelayer (4′) does not contain plasticizer.

According to another embodiment, the present invention relates to amultilayer tubular structure (MLT) as defined above, in which thestructure comprises the layers in the following order:

(3′)//(3)//(4′)//(1)//(4)//(2) in which the layers (3) and (3′) are asdefined above, the layer (2) and (4) containing no more than 1.5% byweight of plasticizer relative to the total weight of the composition ofeach layer, in particular the layer (2) and/or (4) does not (do not)contain plasticizer, the layer (4′) comprising plasticizer, inparticular the layer (4′) does not contain plasticizer.

In particular, said layer (3′) of the above six-layer structure isplasticized, said plasticizer being in particular present in aproportion of 1.5% to 15% by weight relative to the total weight of thecomposition of said layer, the thickness of said layer (3′) preferablyrepresents up to 20% of the total thickness of the tubular structure, inparticular up to 200 μm, in particular the layer (3′) is the outermostlayer and is the only plasticized layer, the layer(s) (3) containing noplasticizer.

According to another aspect, the present invention relates to the use ofa multilayer tubular structure MLT as defined above for the transport offuels, in particular of petrol.

According to another aspect, the present invention relates to the use ofa multilayer tubular structure MLT as defined above, to comply with anextractables test, said test consisting especially in filling saidmultilayer tubular structure MLT with alcohol-containing petrol of FAM Btype and in heating everything at 60° C. for 96 hours, then in emptyingit by filtering it into a beaker, then in leaving the filtrate of thebeaker to evaporate at room temperature, to finally weigh this residue,the proportion of which must be less than or equal to 6 g/m2 of tubeinner surface area.

All the variants described for the multilayer tubular structure MLTapply here for the use of said multilayer tubular structure MLT forcomplying with said extractables test.

An extractables value of less than or equal to approximately 6 g/m2 oftube inner surface area indicates that the proportion of extractables isvery low and will thus avoid clogging the injectors.

EXAMPLES

The invention will now be described in more detail by means of thefollowing non-limiting examples.

The following structures were prepared by extrusion:

The multilayer tubes are produced by coextrusion. A McNeil multilayerextrusion industrial line is used, equipped with 5 extruders connectedto a multilayer extrusion head with spiral mandrels.

The screws used are single extrusion screws having screw profilesadapted to the polyamides. In addition to the 5 extruders and themultilayer extrusion head, the extrusion line comprises:

-   -   a die-punch assembly, located at the end of the coextrusion        head; the inside diameter of the die and the outside diameter of        the punch are chosen as a function of the structure to be made        and of the materials of which it is composed, and also as a        function of dimensions of the pipe and of the line speed;    -   a vacuum tank with an adjustable level of vacuum. In this tank        circulates water maintained in general at 20° C., into which is        immersed a gauge for conforming the tube to its final        dimensions. The diameter of the gauge is adapted to the        dimensions of the tube to be made, typically from 8.5 to 10 mm        for a tube with an outside diameter of 8 mm and a thickness of 1        mm;    -   a succession of cooling tanks in which water is maintained at        about 20° C., for cooling the tube along the path from the        drawing head to the drawing bench;    -   a diameter measurer;    -   a drawing bench.

The configuration with 5 extruders is used to make tubes ranging from 2layers to 5 layers. In the case of the structures in which the number oflayers is less than 5, several extruders are then fed with the samematerial.

In the case of the structures comprising 6 layers, an additionalextruder is connected and a spiral mandrel is added to the existinghead, with a view to producing the inner layer in contact with thefluid.

Before the tests, in order to ensure the best properties for the tubeand a good extrusion quality, it is verified that the extruded materialshave a residual moisture content before extrusion of less than 0.08%. Ifthis is not the case, an additional step of drying the material beforethe tests is carried out, generally in a vacuum dryer, overnight at 80°C.

The tubes, which satisfy the characteristics described in the presentpatent application, were taken, after stabilization of the extrusionparameters, the nominal dimensions of the tubes no longer changing overtime. The diameter is monitored by a laser diameter measurer installedat the end of the line.

Generally, the line speed is typically 20 m/min. It generally variesbetween 5 and 100 m/min.

The speed of the extruder screws depends on the thickness of the layerand on the diameter of the screw, as is known to those skilled in theart.

In general, the temperature of the extruders and tools (head and joint)should be set so as to be sufficiently higher than the melting point ofthe compositions under consideration, such that they remain in themolten state, thus preventing them from solidifying and blocking themachine.

The tubular structures were tested on different parameters (Table I).

The amount of extractables was determined and the barrier, and alsoimpact and bursting strength properties were evaluated. Table IIindicates the tests used and the classification of the results.

TABLE I Bursting strength Examples and counter-examples ExtractablesBarrier Impact (1) Counter-example 1: >50 Good Good GoodPA12-TL/binder/EVOH/binder/PA12-TL 400/50/150/50/350 μm Counter-example2: >50 Good Good Good PA12-TL/binder/EVOH/binder- Noplast/PA12-TL400/50/150/50/350 μm Counter-example 3: >50 Good Good GoodPA12-NoPlast/binder-NoPlast/ EVOH/binder-NoPlast/PA12-TL400/50/150/50/350 μm Counter-example 4: >30 Good Good GoodPA12-NoPlast/binder-NoPlast/ EVOH/binder-no-plast/PA11-TL400/50/150/50/350 μm Counter-example 5: >30 Good Good GoodPA12-NoPlast/binder-NoPlast/ EVOH/binder-no-plast/PA610- TL400/50/150/50/350 μm Ex.1: PA12-NoPlast/binder- <5.5 Good Good GoodNoPlast/EVOH/binder-NoPlast/PA610- NoPlast 400/50/150/50/350 μm Ex.2:PA12-NoPlast/binder- <6 Good Good GoodNoPlast/EVOH/binder-NoPlast/PA612- NoPlast 400/50/150/50/350 μm Ex. 3PA12-NoPlast/binder- <6 Good Good GoodNoPlast/EVOH/binder-NoPlast/coPA612- 6T-NoPlast 400/50/150/50/350 μm Ex.4 PA12-NoPlast/binder- <6 Good Good Good NoPlast/EVOH/PA6-NoPlast400/50/150/400 μm Ex. 5 PA12-NoPlast/binder- <5.5 Good Good GoodNoPlast/EVOH/PA610-NoPlast 400/50/150/400 μm Ex. 6PA610-NoPlast/EVOH/PA610- <5.5 Good Borderline Very NoPlast 450/150/400μm good Ex. 7 PA11-TL/PA610- <5.5 Good Good VeryNoPlast/EVOH/PA610-NoPlast good 150/300/150/400 μm Ex. 8PA11-TL/PA610-NoPlast/ <5.5 Good Very good VeryEVOH/PA610-NoPlast/PA11-NoPlast good 150/300/150/250/150 μm Ex. 9PA12-NoPlast/binder- <4.5 Good Borderline GoodNoPlast/EVOH/binder-NoPlast/ PA610-NoPlast 550/50/150/50/200 μm Ex. 10PA12-NoPlast/binder- <5.5 Very good Good Good NoPlast/EVOH24/binder-NoPlast/PA610-NoPlast 400/50/150/50/350 μm Ex. 11 PA12-NoPlast/binder-<4.5 Good Good Good NoPlast/EVOHhi/binder- NoPlast/PA610-NoPlast(550/50/150/50/200 μm) Ex. 12 PA12-TL/binder/ <5.5 Good Good GoodEVOHhi/binder-NoPlast/ PA610-NoPlast 550/50/150/50/200 μm Ex. 13PA12-TL/binder2/ <5.5 Good Good Good EVOHhi/binder2-NoPlast/PA610-NoPlast 550/50/150/50/200 μm Ex. 14 PA12-TL/binder/ <6 Good GoodGood EVOHhi/binder-NoPlast/ PA612-NoPlast 550/50/150/50/200 μm Ex. 15PA12-TL/PPA1OT/ <5 Borderline Good Good PA11-NoPlast 600/250/150 μm Ex.16 PA12-TL/binder/EVOH/PA610- <3 Very good Good Very NoPlast/PPA1OT good350/50/100/400/100 μm Ex. 17 PA12-TL/binder/EVOH/PPA10T/ <5.5 Very goodGood Very PA610-NoPlast good 350/50/100/100/400 μm Ex. 18PA12-TL/binder/EVOH/PA610- <3 Very good Good Very NoPlast/EFEPc good350/50/100/400/100 μm Ex. 19 PA11-TL/PA610-NoPlast/ <5.5 Good BorderlineVery EVOH/PA610-NoPlast/PA11cond- good NoPlast 400/50/150/300/100 μm (1)Bursting strength is the bursting strength after at least 96 h with FAMB biofuel inside; thus, a sufficiently high value to withstand thepressure is sought.

TABLE II Very good Good Borderline Poor Properties (VG) (G) (QG) (P)CE10 bio-petrol barrier, <0.2 0.2-1 1-3 >3 60° C. g/m² · 24 h, 150 μmthick barrier Extractables: <4.5 g/m² 4.5-5.5 g/m² 5.5-6 g/m² >6 g/m²this test consisting of of tube of tube of tube of tube a tube filledwith surface surface surface surface alcohol-containing area area areaarea petrol of FAM B type (inner (inner (inner (inner at 60° C. for 96hours, surface surface surface surface then emptied and area inn) areainn) area inn) area inn) filtered into a beaker, which is then left toevaporate and the residue of which is weighed, the latter having to beless than or equal to 6 g/m² (of tube inner surface area). Impact VW-40°C. <4% <11% <21% >22% Standard VW breakage breakage breakage breakageTL52435 2010 Bursting strength after >30 30-27 27-25 <25 ageing,standard VW N/mm² N/mm² N/mm² N/mm² TL52435 2010 (MPa) (MPa) (MPa) (MPa)The measurements of permeability to petrols (bio-petrol barrier) aredetermined at 60° C. according to a gravimetric method with CE10:isooctane/toluene/ethanol=45/45/10 vol %.The instantaneous permeability is zero during the induction period, thenit gradually increases up to an equilibrium value which corresponds tothe permeability value under continuous operating conditions. Thisvalue, obtained under continuous operating conditions, is considered tobe the permeability of the material.CompositionsPA12-TL: denotes a composition based on polyamide 12, of Mn(number-average molecular weight) 35 000, containing 6% of BBSA (benzylbutyl sulphonamide) plasticizer and 6% of anhydride-functionalized EPRExxelor VA1801 (Exxon), and on 1.2% of organic stabilizers (consistingof 0.8% of phenol Lowinox 44B25 from Great Lakes, of 0.2% of phosphiteIrgafos 168 from Ciba, of 0.2% of UV stabilizer Tinuvin 312 from Ciba).The melting point of this composition is 175° C.PA12-NoPlast=PA12-TL without plasticizer (the latter is replaced by thesame % of PA12)PA11-TL: denotes a composition based on polyamide 11, of Mn(number-average molecular weight) 29 000, containing 5% of BBSA (benzylbutyl sulphonamide) plasticizer, 6% of impact modifier of ethylene/ethylacrylate/anhydride type in the weight ratio 68.5/30/1.5 (MFI 6 at 190°C. under 2.16 kg), and on 1.2% of organic stabilizers (consisting of0.8% of phenol Lowinox 44B25 from Great Lakes, of 0.2% of phosphiteIrgafos 168 from Ciba, of 0.2% of UV stabilizer Tinuvin 312 from Ciba).The melting point of this composition is 185° C.PA11-NoPlast=PA11-TL without plasticizer (the latter is replaced byPA11)PA610-TL=PA610+12% impact modifier EPR1+organic stabilizer+10%plasticizerPA610-NoPlast=PA610-TL without plasticizer (the latter is replaced byPA610)PA612-TL=PA612+12% impact modifier EPR1+organic stabilizer+9%plasticizerPA612-NoPlast=PA612-TL without plasticizer (the latter is replaced byPA612)PA6-TL=PA6+12% impact modifier EPR1+organic stabilizer+12% plasticizerPA6-NoPlast=PA6-TL without plasticizer (the latter is replaced by PA6)

-   -   PA12: Polyamide 12, of Mn (number-average molecular weight)        35 000. The melting point is 178° C., the enthalpy of fusion        thereof is 54 kJ/m²    -   PA11: Polyamide 11, of Mn (number-average molecular weight)        29 000. The melting point is 190° C., the enthalpy of fusion        thereof is 56 kJ/m²    -   PA610: Polyamide 6.10, of Mn (number-average molecular weight)        30 000. The melting point is 223° C., the enthalpy of fusion        thereof is 61 kJ/m²    -   PA612: Polyamide 6.12, of Mn (number-average molecular weight)        29 000. The melting point is 218° C., the enthalpy of fusion        thereof is 67 kJ/m²    -   PA6: Polyamide 6, of Mn (number-average molecular weight)        28 000. The melting point is 220° C., the enthalpy of fusion        thereof is 68 kJ/m²    -   EPR1: Denotes an EPR functionalized with an anhydride-functional        reactive group (at 0.5-1% by weight), MFI 9 (at 230° C., under        10 kg), of Exxellor VA1801 type from Exxon.        Organic stabilizer=1.2% of organic stabilizers consisting of        0.8% of phenol (Lowinox 44B25 from Great Lakes), of 0.2% of        phosphite (Irgafos 168 from Ciba), of 0.2% of UV stabilizer        (Tinuvin 312 from Ciba).        Plasticizer=BBSA (benzyl butyl sulphonamide)        coPA612-6T-NoPlast=coPA6.12/6.T with 20 mol % of 6.T (thus 80        mol % of 6.12) (this coPA has MFI 235° C., 5 kg=(T° melting=200°        C.)+20% EPR1+orga. stab.)        PPA10T=coPA10.T/6.T, of 60/40 molar ratio, T melting 280° C.+18%        EPR1+orga. stab.        PA11cond-noplast=PA11, of Mn 15 000+9% EPR1+22% of Ensaco 200        type carbon black        Binder=Composition based on 43.8% PA612 (as defined elsewhere),        on 25% of PA6 (as defined elsewhere) and on 20% of EPR1 type        impact modifier, and on 1.2% of organic stabilizers (consisting        of 0.8% of phenol Lowinox 44B25 from Great Lakes, of 0.2% of        phosphite Irgafos 168 from Ciba, of 0.2% of UV stabilizer        Tinuvin 312 from Ciba), and on 10% of BBSA (benzyl butyl        sulphonamide) plasticizer.        Binder-NoPlast=Composition based on 48.8% PA612 (as defined        elsewhere), on 30% of PA6 (as defined elsewhere) and on 20% of        EPR1 type impact modifier, and on 1.2% of organic stabilizers        (consisting of 0.8% of phenol Lowinox 44B25 from Great Lakes, of        0.2% of phosphite Irgafos 168 from Ciba, of 0.2% of UV        stabilizer Tinuvin 312 from Ciba).        Binder2=Composition based on 43.8% PA610 (as defined elsewhere),        on 25% of PA6 (as defined elsewhere) and on 20% of EPR1 type        impact modifier, and on 1.2% of organic stabilizers (consisting        of 0.8% of phenol Lowinox 44B25 from Great Lakes, of 0.2% of        phosphite Irgafos 168 from Ciba, of 0.2% of UV stabilizer        Tinuvin 312 from Ciba), and on 10% of BBSA (benzyl butyl        sulphonamide) plasticizer.        Binder2-NoPlast=Composition based on 48.8% PA610 (as defined        elsewhere), on 30% of PA6 (as defined elsewhere) and on 20% of        EPR1 type impact modifier, and on 1.2% of organic stabilizers        (consisting of 0.8% of phenol Lowinox 44B25 from Great Lakes, of        0.2% of phosphite Irgafos 168 from Ciba, of 0.2% of UV        stabilizer Tinuvin 312 from Ciba).        EVOH=EVOH containing 32% of ethylene, EVAL FP101B type (Eval)        EVOH24=EVOH containing 24% of ethylene, EVAL M100B type (Eval)        EVOHhi=EVOH containing 27% of ethylene and impact modifier, EVAL        LA170B type (Eval)        PPA10T/6T=coPA10.T/6.T with 40 mol % of 6.T (of MFI 300° C., 5        kg=8, and of 1° C. melting 280° C.)+15% of EPR1+orga. stab.)        EFEPc=functionalized and conductive EFEP of Neoflon RP5000AS        type, from Daikin        Binder PA610+PA6. Denotes a composition based on PA612 (of Mn        29000, and as defined elsewhere) and on 36% of PA6 (of Mn 28        000, and as defined elsewhere), and on 1.2% of organic        stabilizers (consisting of 0.8% of phenol Lowinox 44B25 from        Great Lakes, of 0.2% of phosphite Irgafos 168 from Ciba, of 0.2%        of UV stabilizer Tinuvin 312 from Ciba).        The structures having layers not containing plasticizer, located        below the barrier and especially in contact with the fluid, have        excellent results in the extractables test, much better than the        counter-examples in which the layer in contact with the fluid is        plasticized.

The invention claimed is:
 1. A multilayer tubular structure (MLT)intended for the transport of fuels, wherein the MLT consists of thefollowing five layers, from the outside inwards: a first outer layer(3), a second outer layer (3′) in contact with and inside of the firstouter layer (3), a barrier layer (1) in contact with and inside of thesecond outer layer (3′), a first inner layer (2) in contact with andinside of the barrier layer (1), and a second inner layer (2′) incontact with and inside of the first inner layer (2), wherein: thebarrier layer (1) comprises EVOH or comprises PPA; the first inner layer(2) comprises a polyamide, where the polyamide is chosen from the groupconsisting of PA6, PA66, PA6/66, PA610, PA612, PA1012, the correspondingcopolyamides thereof, and mixtures thereof; the second outer layer (3′)comprises at least one aliphatic polyamide of aliphatic type orcomprising more than 75% of aliphatic units; the first outer layer (3)comprises plasticizer in a proportion from 1.5% to 15% by weightrelative to the total weight of the composition of the first outerlayer, and the thickness of the first outer layer comprises up to 20% ofthe total thickness of the multilayer tubular structure; the secondinner layer (2′) comprises a polyamide as defined for the first innerlayer (2) but different from that of the first inner layer (2); and thesecond outer layer (3′), the barrier layer (1), the first inner layer(2) and the second inner layer (2′) do not comprise plasticizer.
 2. TheMLT according to claim 1, wherein the polyamide of the second outerlayer (3′) is a totally aliphatic polyamide.
 3. The MLT according toclaim 1, wherein the barrier layer (1) comprises EVOH, and the EVOHcomprises up to 27% of ethylene.
 4. The MLT according to claim 1,wherein the barrier layer (1) comprises EVOH, and the EVOH furthercomprises an impact modifier.
 5. The MLT according to claim 1, whereinthe polyamide of the second outer layer (3′) is chosen from the groupconsisting of PA11, PA12, PA610, PA612, PA1012, the correspondingcopolyamides thereof, and mixtures thereof.
 6. The MLT according claim1, wherein the polyamide of the second outer layer (3′) is chosen fromthe group consisting of PA11, PA12, PA610, PA12, PA1012, thecorresponding copolyamides and mixtures therefore.
 7. The MLT accordingto claim 1, wherein the polyamide of at least one of the layers is aconductive polyamide.
 8. The MLT according to claim 1, wherein at leastone of the first inner layer (2) and the second outer layer (3′),comprises at least one of an impact modifier and at least one otheradditive.
 9. The MLT according to claim 1, wherein the at least onealiphatic polyamide of the first inner layer (2) includes two polyamidesselected from the group consisting of: PA6, PA66, PA6/66, PA610, PA612and PA1012.
 10. A method of transporting fuel, wherein the methodcomprises filling the MLT according to claim 1 with fuel and thentransporting the MLT from one location to another.